Organic light emitting diodes (OLED) have been increasingly used as current-type light-emitting devices in high-performance Active Matrix Organic Light Emitting Diode displays. With increasing of display size, conventional passive matrix organic light emitting diode displays require a shorter driving time for a single pixel, and thus require an increased transient current, which causes increased power consumption. Meanwhile, a voltage drop on a line of nanometer indium tin oxide will be too large when a large current is applied, such that an operating voltage of OLED is too high and efficiency of OLED is decreased. The currents for OLEDs are input to active-matrix organic light-emitting diode displays when switching transistors are scanned progressively, which can solve the above problems well.
In design of an AMOLED backboard, a main problem to be solved is non-uniformity of luminance among compensating circuits for respective pixel units of AMOLED.
First, for AMOLED, pixel circuits are constituted by thin film transistors to supply currents for driving light emitting devices, respectively. In prior art, Low-temperature poly-silicon thin film transistors (LTPS TFT) or oxide thin film transistor (Oxide TFT) are mostly adopted. Compared to a general amorphous silicon thin film transistor (amorphous-Si TFT), LTPS TFT and Oxide TFT have higher mobility and more stable characteristics, and thus are more suitable for AMOLED display. However, due to limitations of the crystallization process, LTPS TFTs produced on a large-area glass substrate often have non-uniformity on electrical parameters such as threshold voltage, mobility and the like, and such non-uniformity may cause driving current difference and luminance difference among OLED devices, that is, a mum phenomenon occurs, which may be perceived by human eyes. Although process of Oxide TFTs shows a better uniformity, similar to a-Si TFTs, a threshold voltage of Oxide TFT may drift under a high temperature or supplied with a voltage for a long time. Due to different images as displayed, drifts of threshold voltages of TFTs in respective areas on a panel may be different from each other, which may cause display luminance difference, such a display luminance difference often renders an image sticking phenomenon since such a display luminance difference has a relation to a previously displayed image.
Second, in large-size display applications, since a certain resistance exists in a power supply line on the backboard, and driving currents for all pixels are supplied from an ARVDD power supply, a supply voltage for an area near to a location of the ARVDD power supply is higher than a supply voltage for an area far from the location, such a phenomenon is known as a voltage drop of the power supply (IR Drop). As the voltage of the ARVDD power supply has a relation to currents in different areas, IR drop may also cause driving current difference among different areas, and thus a mum phenomenon appears during display. LTPS process for constructing pixel units by adopting P-type TFTs is sensitive to such an IP drop since a storage capacitor therein is connected between the ARVDD and a gate of TFT, and thus voltage variation of ARVDD may directly affect a gate voltage Vgs of the driving TFT.
Third, the non-uniformity of the electrical characteristics of the light-emitting devices may also be resulted from non-uniform thickness of the mask during an evaporation process. For the a-Si or Oxide TFT process constructing pixel units by adopting N-type TFTs, a storage capacitor therein is connected between a gate of the driving TFT and an anode of the light-emitting device, if voltages at the first terminals of the light-emitting devices of respective pixels are different when a data voltage is transmitted to the gates, voltages Vgs actually applied to the gates of TFTs may be different, so that display luminance are different due to different driving currents.
Therefore, in order to solve the above problems, there is a need for a pixel circuit and a method for driving the same.